This invention relates to optical polarisation state controllers, and in particular to such controllers in which the optical path lies in single mode optical fibre. The control of the state of polarisation (SOP) of an optical signal is an essential part of many optical sensor and coherent optics communications systems.
It is known, for instance from a paper by H. C. Lefevre entitled `Single-Mode Fibre Fractional Wave Devices and Polarisation Controllers` (Electronics Letters Sept. 25, 1980 Vol. 16 No. 20 pages 778-780), that a controllable amount of stress-induced birefringence can be imparted to a length of single mode fibre by stretching, bending or twisting a portion of that length, and that such effects can be used as the basis of an SOP controller to provide any desired output SOP from the fibre for an input of defined SOP. The above-referenced paper is particularly directed to polarisation controllers in which adjustment of SOP is achieved by inducing a specific amount of linear birefringence into a length of single mode fibre by winding it into a planar coil with substantially co-linear ends, and then modifying the resulting birefringence by rotating the plane of the coil in such a way as to impart a controlled amount of twist into those ends. With reference to its FIG. 3, the paper described a tandem arrangement of two .lambda./4 and a .lambda./2 coil. The adjustment of the orientation of the two .lambda./4 coils may be used to convert any given input SOP to a linear SOP, and then the adjustment of the orientation of the .lambda./2 coil can be used to set the output to any chosen specific orientation of linear SOP.
The mounting of the coils in a manner providing easy adjustment of the orientation of each imposes some difficulty, particularly when the adjustment is not performed manually, but is performed by transducers designed to enable the output SOP to be changed in any specifically desired time-varying manner.